The mouse mammary tumor and its associated virus, mouse mammary tumor virus (MMTV), have been chosen as a model system to test the feasibility of employing plasma levels of viral proteins to detect the presence and extent of murine mammary neoplasia. Work with mice suffering early onset of tumor has proven that a major virion protein, gp52, is a useful systemic marker for the presence of tumor. The proposed studies seek to extend our understanding of MMTV glycoprotein expression and its relation to tumor by focusing on mice with late-occurring mammary tumors (MMTV-L-related). Well-characterized monoclonal antibodies will be used in isotopic staphylococcal protein A tests to characterize the presence and relative abundance of unique (strain-specific) and shared gp52 determinants exposed on the surface of C3Hf, C3H, and GR mammary tumor cells. The results obtained will provide detailed knowledge of how gp52 is presented to the immune system. This knowledge will help to explain the many gp52-directed immune responses of tumor-bearing mice and the failure of immune surveillance in mice carrying different MMTV strains. Assays developed with monoclonal antibodies will permit the MMTV strain-relatedness of plasma gp52 determinants from mammary and nonmammary tumor-bearing mice to be determined. This study will indicate if gp52 detected in plasma can be predominately attributed to endogenous or exogenous MMTV expression. The virion envelope glycoproteins, gp52 and gp36, will be measured by RIA in the plasma of mice with both early and late-occurring mammary tumors. This study will indicate which virion glycoprotein is present in greatest abundance and is most useful as a diagnostic marker for both early-and late-occurring tumors. Assays will be used to detect the presence of the glycoprotein precursor, gPr75, and its contribution to the plasma pool of gp52 determinants. Although previous work has centered on the release of virion-associated antigens from mammary tumor cells, cultures of C3Hf mammary tumor cells will be used to aid in our understanding of those processes and factors that affect the release of nonvirion-associated viral antigens. (2)